Liquid crystal displays (LCDs) have been employed in greater numbers since RCA made the first experimental LCD in 1968 and are widely used in wrist watches and flat panel displays for calculators, and also in computer and TV screens. However, liquid crystal materials emit no light of their own and thus require an external light source; for example, many laptop computer LCD panels are lit by fluorescent tubes, such as cold cathode fluorescent lamps, which are associated therewith. Subsequently, more economical discrete light-emitting devices and displays have been made possible through the phenomenon of electroluminescence (EL), and flat panel displays on glass have become commercial and are termed light-emitting devices (LEDs). More recently, organic materials are beginning to be employed to replace conventional inorganic materials previously used in LEDs, and this new class of materials has become referred to organic LEDs or OLEDs. There are projects presently funded by the Defense Advanced Research Projects Agency (DARPA) to develop flexible OLEDs (FOLEDs) that will bend and roll-up.
Organic luminescent cells are generally constructed as a laminate of organic EL materials and electrodes of opposite polarity, and one is broadly shown in U.S. Pat. No. 4,356,429, issued in 1982, which includes a hole injector zone between the EL and the anode, which was commonly indium tin oxide (ITO). Since that time, many improvements have been made.
Basically, OLEDs are presently often made by placing an organic light-emitting material between a layer of a conductive material (ETL) that can inject electrons and a layer of a conductive material (HTL) that can inject holes. This arrangement is placed between flanking outer layers of conductive material that serve as electrodes so that, when a voltage is applied between such outer electrode layers, electrons from one layer combine with holes from the other, releasing energy as light, i.e. producing electroluminescence (EL). It is also possible to produce EL emission from certain ETLs and HTLs without the need for a separate EL layer, or to omit the ETL layer. Such devices are described in Burroughes et al., Nature 347,539 (1990) and by Braun and Heeger, Applied Physics Letters 58, 1982 (1991).
Commonly, such OLEDs were first deposited onto a transparent glass substrate through which the display might be viewed, and these devices have now grown so as to constitute well known multilayer devices where each layer serves a specific function. To make these devices lighter, thinner and more rugged and also to provide flexibility where desired in the ultimate device, glass in OLEDs has now frequently been replaced with a transparent polymeric substrate, and such a substrate may be coated with a transparent conducting material, one or more organic or polymeric layers adjacent an electroluminescent layer and a metal cathode layer. The organic layers provide charge injection and transport from both electrodes into the EL layer where the charge is recombined and emits light. Depending upon the particular design, one or multiple organic hole transporting layers (HTLS) may be provided between the transparent conducting anode and the EL layer, and one or multiple electron injection and transporting layers may be provided between the cathode and the EL layer. However, as a result of these continuing developments, it has been recognized that certain organic materials and essentially cathode materials are particularly sensitive to oxidation and/or degradation upon exposure to atmospheric oxygen and humidity.
For the same reasons of desiring to reduce weight and thickness, increase durability and impart flexibility, polymer layers have also begun to replace glass in LCDs. As a result, degradation of the liquid crystal materials over time has become of more concern because LCDs can also experience similar problems upon exposure to the environment. For example, U.S. Pat. No. 4,709,991 discusses the need to protect LCDs from exposure to oxygen and water vapor.
To prevent OLEDs and LCDs from degrading in their performance as a result of the intrusion of oxygen and/or moisture through transparent polymers being used as a substrate, e.g. polyesters, such as polyethylene terephthalate (PET) or polyethylene naphthalate (PEN), it has been necessary to apply sealing layers to these polymers that serve as barriers to oxygen/moisture flow therethrough. One method of providing such barrier layer protection is through the application of a multitude of layers deposited from a high vacuum, vapor atmosphere, and it has been sometimes referred to as the PML technique, i.e. for polymer multilayer technique (see U.S. Pat. No. 5,032,461). U.S. Pat. No. 5,757,126 also describes depositing a multilayer overcoating upon a plastic substrate using web processing equipment by applying alternating layers of polymers and inorganic materials to a flexible web of plastic to create a protected substrate onto which an OLED can be deposited. When employing monomer flash evaporation, as taught in the '461 patent, the polymer is generally supplied to the substrate surface in the form of droplets of a monomer, about 10 microns or less in size, with the deposited layer of droplets then being polymerized to a film by UV radiation curing, E-beam curing or the like. Metal and/or metal oxide layers have also been deposited on such a polymer film using conventional electron beam vaporization of the type generally employed to deposit metalizing films in a vacuum environment and plasma enhanced chemical vapor deposition (PECVD). While resistance to the passage of oxygen and/or water is substantially improved, compared to uncoated PET substrates, OLEDs made using this basic technique still have limited lifetimes; accordingly, further improvements have continued to be sought.
Although it has been alleged that an improvement in barrier performance may result from the use of plasma-enhanced chemical vapor deposition utilizing an electron cyclotron resonance source, such is considered to be an expensive option and one that may not be truly commercially feasible.
Generally, it is the object of the invention to increase the lifetime of an OLED or LCD, and particularly a flexible LED or LCD or the like, by providing barrier material that has improved resistance to the transmission of oxygen and moisture from the atmosphere which would otherwise cause the slow degradation of an organic color-generating and/or light-emitting layer or other susceptible component, particularly cathode material, which barrier has advantageous overall physical properties. It is a further object of the invention to provide improved methods for making such barrier material, including flexible barrier material, which is useful to prevent the passage of water and oxygen to a display device, such as one which employs a susceptible cathode or one which incorporates organic electroluminescent display material on a flexible plastic substrate.